Microbial microstratification, inorganic carbon photoassimilation and darkcarbon fixation at the chemocline of the meromictic Lake Cadagno (Switzerland) and its relevance to the food web

The microstratification of the microbial community at the chemocline of Lak e Cadagno and the associated inorganic carbon fixation activity was studied by fine layer sampling. A deep chlorophyll maximum caused by diatoms overl ying Cryptomonas was found at the upper edge of the chemocline. A high popu lation density of phototrophic sulphur bacteria, mainly Amoebobacter cf. pu rpureus, occurred closely below the oxic-anoxic boundary. Despite the small fraction of total lake volume represented by the chemocline, half of the t otal carbon photoassimilation of the lake occurred within the chemocline wi th approximately equal contributions by oxygenic and anoxygenic phototrophs . Rates of dark carbon fixation in the chemocline were even higher than rat es of photoassimilation, especially at the depths where oxygen and sulphide coexisted during part of the day. These results indicate a substantial con tribution by chemolithotrophic organisms to the carbon cycle in Lake Cadagn o. Analysis of stable carbon isotopes suggests that zooplankton may obtain as much as half of its carbon at the chemocline, indicating a strong link b etween production in anoxic waters and the food web in the oxic part of the lake.